1. Field of the Invention
The present invention relates to a wire bonding method and apparatus for connecting a first bonding point and a second bonding point with a wire, and more particularly to a tail setting method and apparatus in such a wire bonding method and apparatus.
2. Prior Art
Various types of wire bonding methods have been proposed. FIG. 5 shows the most common method.
In FIG. 5, first, in step (a), a ball 2a is formed by the discharge of an electric torch 3 on a wire 2 that extends out of the lower end of the capillary 1. Afterward, the electric torch 3 is moved in the direction indicated by arrow. Next, in step (b), the capillary 1 is moved to a point above the first bonding point 4a of a semiconductor chip 4. Then, in step (c), the capillary 1 is lowered, and the ball 2a on the tip end of the wire 2 is bonded to the first bonding point 4a. 
Afterward, in step (d), the capillary 1 is raised. Then, in step (e), the capillary 1 is moved to a point above the second bonding point 5a of a lead 5. Next, in step (f), the capillary 1 is lowered, and the wire 2 is bonded to the second bonding point 5a. Subsequently, after the capillary 1 has been raised to a fixed position, a damper 6 is closed, and the capillary 1 and damper 6 are raised together so that the wire 2 is cut from the root portion of the second bonding point 5a in step (g), thus causing a tail 2b to be formed at the lower end of the capillary 1.
As a result, one wire connection is completed.
Japanese Patent Application Laid-Open (Kokai) Nos. S57-87143 and H1-26531 disclose wire bonding methods of the type described above. In Japanese Patent Application Laid-Open (Kokai) No. S57-87143, the capillary is moved along an upwardly rounded arc trajectory at the uppermost point of the movement of the capillary between the first bonding point and the second bonding point. In Japanese Patent Application Laid-Open (Kokai) No. H1-26531, after the bonding to the first bonding point, the capillary is moved above the first bonding point and toward the second bonding point along an arc trajectory, and then bonding is made to the second bonding point.
The above-described operation in which the wire 2 is cut from the root portion of the second bonding point 5a is accomplished as a result of the damper 6 that is closed at an intermediate point during the upward movement of the capillary 1 and is raised together with the capillary 1. Accordingly, if there is a variation in the amount of opening of the damper 6 depending on the respective dampers 6 attached to the wire bonding apparatus, there will be a time discrepancy at which the damper 6 is closed to clamp or hold the wire 2, even if the timing at which the clamping or holding of the wire 2 is initiated for the purpose of cutting the wire 2 as described above is the same. Consequently, the length of the tail 2b will vary. This will be described in more detail with reference to FIG. 6.
In FIG. 6, the diameter of the wire 2 is, for example, 30 xcexcm, and the opening and closing control per 1 xcexcm of movement of the damper 6 is accomplished by output control at, for example, 0.025 ms. In addition, in FIG. 6, amount of opening of a certain clamper 6A is 80 xcexcm, and the amount of opening of another damper 6B is 60 xcexcm. The amount of closing in a case where the damper 6A whose amount of opening is 80 xcexcm holds a wire 2 that has a diameter of 30 xcexcm is (80xe2x88x9230)=50 xcexcm. Accordingly, the clamping time TA is as follows: TA=50 xcexcmxc3x970.025 ms/xcexcm=1.25 ms. The amount of closing in a case where the damper 6B whose amount of opening is 60 xcexcm holds a wire 2 that has a diameter of 30 xcexcm is (60xe2x88x9230)=30 xcexcm; accordingly, the clamping time TB in this case is as follows: TB=30 xcexcmxc3x970.025 ms/xcexcm=0.75 ms. In other words, the damper 6B holds the wire 2 earlier than the damper 6A by a time of Tb=(1.25xe2x88x920.75)=0.5 ms.
In order to obtain a length of the tail 2b (tail length) of 360 xcexcm, it is sufficient in the case of the damper 6A to initiate the clamping operation at a time of 1.25 ms prior to the time that the capillary 1 reaches 360 xcexcm during the rise of the capillary 1 from the step (f) in FIG. 5. However, if the clamping operation is similarly initiated 1.25 ms in advance for the damper 6B, the position (timing) at which the wire 2 is held or clamped by the damper 6B will be advanced by Tb=0.5 ms, so that the tail length is shortened. For example, in a case where the capillary 1 performs a constant-speed operation at a speed of 72 xcexcm/ms (7.2 xcexcm/pulse), the damper 6B is shortened by a length of 72 (xcexcm/ms)xc3x970.5 ms=36 xcexcm. In other words, assuming that the tail length LA of the damper 6A is 360 xcexcm, the tail length LB of the damper 6B is 360xe2x88x9236=324 xcexcm.
In cases where the damper 6 is, for instance, replaced, it is necessary to adjust the closing timing of the damper 6 (6A or 6B) in the process from the step (f) to step (g) of FIG. 5 in accordance with the amount of opening of the damper 6 (6A or 6B). Conventionally, the following two methods have been used for this. In the first method, the capillary 1 is stopped in a position in which the capillary 1 has been raised by an amount equal to the tail length; then, after the damper 6 is closed, the capillary 1 is again raised. In the second method, the speed at which the damper 6 is raised is slowed beginning at a point immediately prior to a tail position of a specified length, so that the amount of variation in the tail length is reduced.
In the above-described methods, a stopping operation or low-speed operation is performed. Accordingly, the bonding cycle is slowed, and the productivity drops. Furthermore, in the above-described description, the speed at which the capillary 1 is raised is assumed to be 72 xcexcm/ms; in actuality, however, this speed is set at 360 to 720 xcexcm/ms in order to increase the productivity, so that it is desirable that the tail length be stable even under such high-speed conditions.
The object of the present invention is to provide a wire bonding method and apparatus which eliminates differences between individual dampers and obtains a stable tail length without lowering the speed for raising a capillary.
The above object is accomplished by unique steps taken in a wire bonding method in which a wire is bonded to a second bonding point, a capillary is raised by a specified amount, then a damper is closed, and said capillary and said damper are raised together so that the wire is cut from the root portion of the second bonding point, thus causing a tail of the wire used for forming a ball to extend from the lower end of the capillary; and in the present invention,
the capillary is moved to a measurement position above a tail length measuring member after the tail has been extended,
the capillary is then lowered so that the tip end of the tail contacts the tail length measuring member, and the position of the capillary or the distance by which the capillary has been lowered, at the time that electrical continuity is established with the tail length measuring member, is detected, and
the tail length is calculated based upon:
(a) the height level of the tail length measuring member, the position of the capillary prior to the lowering of said capillary for the purpose of tail measurement, and the position of the capillary at the time that the wire contacts the tail length measuring member, or from
(b) the height of the capillary above the tail length measuring member prior to the lowering movement of the capillary for the purpose of tail measurement, and the distance by which the capillary is lowered.
In the above, the tail length measuring member is an electric torch which is used to form a ball on the tail, or a semiconductor chip.
Furthermore, in the present invention, the tail length is calculated by the abovedescribed method, and the clamp timing of the damper is corrected on the basis of the results of this calculation.
Furthermore, the correction of the clamp timing of the damper is performed according to:
the speed at which the capillary is raised at the time that the capillary is raised following bonding to the second bonding point, and
the difference between the measured tail length and a reference tail length.
The above object is accomplished by a unique structure for a wire bonding apparatus in which a wire is bonded to a second bonding point, a capillary is raised by a specified amount, then a damper is closed, and said capillary and said damper are raised together so that the wire is cut from the root portion of the second bonding point, thus causing a tail of the wire used for forming a ball to extend from the lower end of the capillary; and the unique structure of the present invention comprises:
a micro-voltage application circuit which applies a micro-voltage to the wire,
a contact detection section which outputs a detection signal when the capillary from which the tail has been extended is moved to a measurement position above a tail length measuring member and is then lowered so that the tip end of the tail contacts the tail length measuring member, and
a computer which calculates the tail length based upon:
(a) the height level of the tail length measuring member, the position of the capillary prior to the lowering of said capillary for the purpose of tail measurement, and the position of the capillary at the time that the wire contacts the tail length measuring member, or
(b) the height of the capillary above the tail length measuring member prior to the lowering movement of the capillary for the purpose of tail measurement, and the distance by which the capillary is lowered.
In the above structure, the tail length measuring member is an electric torch which is used to form a ball on the tail, or a semiconductor chip.
In addition, the computer calculates the tail length and corrects the clamp timing of the damper on the basis of the results of this calculation.
Furthermore, the correction of the clamp timing of the damper by the computer is performed according to:
the speed at which the capillary is raised at the time that the capillary is raised following bonding to the second bonding point, and
the difference between the measured tail length and a reference tail length.